The increasing popularity of mobile communications has resulted in modifications in antennas to suit the developments in communications devices. Even so, prior to mobile cellular phone service, mobile communications were most often found in use by either hobbyists or professionals who required the communications capability for business purposes. In this regard, most vehicles in which mobile communications systems were installed were not personal or luxury type vehicles.
With the explosion of cellular phone service, however, mobile communications installations have expanded to include almost all types of vehicles and users. Phones are being installed in fleets, executive vehicles, and personal vehicles. With the everincreasing utilization, particularly in the 800 MHz cellular phone environment, antenna technology, and the number of differing environments in which antennas may be installed has increased to a point where the numbers of antennas that might be required to accommodate the possible installation variations and vehicle configurations could become prohibitively expensive for those who have to maintain an inventory of such components.
The advent of cellular radio telephone service has resulted in increases in the number of antennas sold and installed. As a result of the growing awareness of cellular service, interest in private mobile radio systems is also increasing.
From an operational point of view, roof-top antenna installation usually results in the best performance, mainly because of additional height, absence of reflections, an unobstructed view in azimuth, and the symmetry of radiation patterns, all of which contribute to the greatest omnidirectional coverage. However, a roof-top antenna installation is not the automatic choice. Such an installation may be impractical, as in the case of a convertible. It is often undesirable because vehicle owners do not wish to have holes drilled in the roof of their vehicles. Roof construction is making roof-top installations more and more difficult. Roof walls are becoming thinner, ribbed double paneling and molded headliners make it difficult to route cabling and limit natural cable access. When roof-top installations are difficult, they are time-consuming and expensive.
Good mobile performance is the result of a number of factors, one of which is a good omnidirectional radiation pattern. Mounting locations other than roof-tops tend to present some compromises because of lower height and the potentially asymmetrical pattern resulting from vehicle body obstructions. Antennas installed at alternative locations can maintain very nearly as good a performance characteristic as the roof-top installations.
One very popular and successful alternative is an antenna that is mounted on the window of a vehicle. This antenna serves as a practical solution for customers and users who object to drilling holes in the vehicle. The window-mounted antenna is easily and quickly attached to a glass, plastic, or fiberglass surface, e.g., a windshield or rear window, making it possible to select a location based on the simplest path for the cable connection to the radio telephone. Furthermore, when mounted near the top of such a window, the antenna can extend above the adjacent surface and provide a good uniform omnidirectional radiation pattern.
Another excellent alternative to the roof-top installation is an elevated feed point antenna. This antenna includes a lower mast in the form of an isolation skirt or sleeve and a choke contained in the lower mast. The antenna requires no ground plane, and the higher center of radiation achieved by the elevated feed design addresses a major cause of pattern distortion and signal loss, which can be experienced when antennas designed for roof-top installation are mounted at alternative lower locations, such as on trunks or cowls.
Because of the wide variety of vehicle shapes, one of the problems with a trunk or cowl installation is that the antenna is not vertically oriented when installed on a slanted surface. While various adjustable antennas exist, one of the problems in the 800 MHz band is to avoid deterioration of operating characteristics, which is more critical at these frequencies than at the lower frequencies at which antennas have been used in the past. Thus, the use of an antenna properly designed to meet the requirements of a particular mounting location is important at the 800 MHz band.
At these cellular frequencies, some of the mechanical problems of antenna design are reduced. For example, full-length whips are shorter and, therefore, less subject to problems of angular deflection or the necessity to produce antennas in which physical length differs from electrical length, such as is the case for most antennas used in the CB frequency bands. Other problems are more severe.
Thus, although the 800 MHz cellular band antenna looks deceivingly simple, there are a number of factors that must be addressed in order to produce a successful antenna. Noise generation from mechanical components which might go unnoticed at lower frequencies can utterly destroy the integrity of an 800 MHz type antenna that depends on secondary data transmission for proper operation or cell site and channel switching. Mechanical components and connections must be highly resistant to normal vibrations, as well as corrosion, which can lead to noise generation in such duplex communications systems.
High-grade coaxial cable is to be used, and the connections between the cable and the antenna can create trouble spots, particularly where low noise is essential to accurate communications.
Existing elevated feed and other trunk or cowl mount antennas are designed to be installed in the center of the trunk or cowl where a hole can be drilled through the trunk lid or the cowl. Alternatively, there are designs which are designed to clip or be screwed to the edge of a trunk. As indicated above, however, such installations may result in the antenna not being vertically oriented.
It would, therefore, be desirable to be able to have a trunk or cowl mounted antenna, such as an elevated feed antenna, adapted for mounting to a variety of surfaces having different angular orientations, which is adjustable and capable of orienting the antenna in a desired plane, typically the vertical, without producing deterioration in the operating characteristics of the antenna.